Support \tag, \tag*, and \gdef (#1309)

* Tag sketch

* Drop objectAssign; already using Object.assign elsewhere

* Basic \gdef support

* Bug fix

* Finish \tag

* MathML numbers equations with <mlabeledtr>

* Fix flow bugs

* \gdef tests

* Add basic \tag tests

* Screenshot test for \tag

* \tag* test

* Add missing file

* Bug fix screenshot

* Major refactor

* Represent tag at top level of parse tree, requiring less hackery
  * No more \@tag function; it was essentially just doing \text
* Wrap tag in group so e.g. ( and ) are formatted the same
* Add `feed` method to MacroExpander for multiple inputs (for tag)
* Bug fixes in buildHTML, makeTextRow, _getBuilt (for display mode)
* Remove excess <mrow> wrapper when unnecessary

* Update screenshot from tag being wrapped in group

* Add maxExpand limit

README.md

@@ -54,7 +54,8 @@ Make sure to include the CSS and font files, but there is no need to include the
 
 Any HTML generated by KaTeX *should* be safe from `<script>` or other code
 injection attacks.
-(See `maxSize` below for preventing large width/height visual affronts.)
+(See `maxSize` below for preventing large width/height visual affronts,
+and see `maxExpand` below for preventing infinite macro loop attacks.)
 Of course, it is always a good idea to sanitize the HTML, though you will need
 a rather generous whitelist (including some of SVG and MathML) to support 
 all of KaTeX.
@@ -82,7 +83,8 @@ You can provide an object of options as the last argument to `katex.render` and
 - `errorColor`: `string`. A color string given in the format `"#XXX"` or `"#XXXXXX"`. This option determines the color that unsupported commands and invalid LaTeX are rendered in when `throwOnError` is set to `false`. (default: `#cc0000`)
 - `macros`: `object`. A collection of custom macros. Each macro is a property with a name like `\name` (written `"\\name"` in JavaScript) which maps to a string that describes the expansion of the macro. Single-character keys can also be included in which case the character will be redefined as the given macro (similar to TeX active characters).
 - `colorIsTextColor`: `boolean`. If `true`, `\color` will work like LaTeX's `\textcolor`, and take two arguments (e.g., `\color{blue}{hello}`), which restores the old behavior of KaTeX (pre-0.8.0). If `false` (the default), `\color` will work like LaTeX's `\color`, and take one argument (e.g., `\color{blue}hello`).  In both cases, `\textcolor` works as in LaTeX (e.g., `\textcolor{blue}{hello}`).
-- `maxSize`: `number`. If non-zero, all user-specified sizes, e.g. in `\rule{500em}{500em}`, will be capped to `maxSize` ems. Otherwise, users can make elements and spaces arbitrarily large (the default behavior).
+- `maxSize`: `number`. All user-specified sizes, e.g. in `\rule{500em}{500em}`, will be capped to `maxSize` ems. If set to `Infinity` (the default), users can make elements and spaces arbitrarily large.
+- `maxExpand`: `number`. Limit the number of macro expansions to the specified number, to prevent e.g. infinite macro loops. If set to `Infinity` (the default), the macro expander will try to fully expand as in LaTeX.
 - `strict`: `boolean` or `string` or `function` (default: `"warn"`). If `false` or `"ignore`", allow features that make writing LaTeX convenient but are not actually supported by (Xe)LaTeX (similar to MathJax). If `true` or `"error"` (LaTeX faithfulness mode), throw an error for any such transgressions. If `"warn"` (the default), warn about such behavior via `console.warn`. Provide a custom function `handler(errorCode, errorMsg, token)` to customize behavior depending on the type of transgression (summarized by the string code `errorCode` and detailed in `errorMsg`); this function can also return `"ignore"`, `"error"`, or `"warn"` to use a built-in behavior.  A list of such features and their `errorCode`s:
   - `"unknownSymbol"`: Use of unknown Unicode symbol, which will likely also
     lead to warnings about missing character metrics, and layouts may be